


One Heart, Two Souls

by DuchessDeeDee



Category: Ao no Exorcist | Blue Exorcist
Genre: Demon heritage messed up their dna, Fluff, I swear, M/M, Soulmates, They're not brothers, post episode 25
Language: English
Status: In-Progress
Published: 2019-01-15
Updated: 2019-01-15
Packaged: 2019-10-10 20:17:41
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,703
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17432831
Author URL: https://archiveofourown.org/users/DuchessDeeDee/pseuds/DuchessDeeDee
Summary: Rin pointed at Yukio. “He started it!”“I did not.”“Well, I’m ending it,” Shura grabbed Rin’s arm, yanking him along with her. The elder whimpered like a kitten and it was sorta cute stop judging him. “Enough, you two, honestly. You’d think you were an old married couple instead of twins.”Rin’s face, having finally faded to its normal pale color, flared up. Yukio wondered if it was too late to throw himself off the roof of True Cross Academy.





	One Heart, Two Souls

**Author's Note:**

> I thought of this and then I couldn't stop I SWEAR THEY'RE NOT BROTHERS. Blame episode 25. I do.

 

“We found the grave,” Yukio stated. 

 

Stated was perhaps an underestimate. Yukio liked to think he kept his emotions under control, but his voice was still hinting on the edge of a growl.

 

Mephisto looked up from the game of cat and mouse he was playing with a green hamster, smiling at the Okumuras. “Oh, did you? I rather hoped so. Those keys have been in my possession for 15 long, long years.”

 

Rin slammed his hands on his desk, blue flames licking his form. “Why didn’t you tell us? What was the point in hiding all of this?”   
  


“Rin!” Yukio grabbed the elder by the arm, slightly bemused as the flames leapt from Rin’s skin to his, and back again.

 

“Oh, do relax,” Mephisto waved a bored hand. “It wasn’t like it was my idea to keep you two in the dark. Your father was as much to blame as I. Weren’t you happy to believe yourselves his children, or children of none with a father figure? Would you have been happy knowing your grandfather tried to burn his own pregnant daughter at the stake, and that Satan was the father? Tell me I wouldn’t still be cleaning up your little rampage if I had told you.”

 

Rin slumped into a conveniently placed armchair, leaning to one side to make room for Yukio.

 

“Ok, fine, you couldn’t tell us before we knew Satan was our father. But then why didn’t you tell Rin when you were both trapped in the Vatican’s prison?” Yukio asked, squeezing into the space between Rin and the armrest. “Why didn’t you tell me when you knew he was coming to find me? I- _ We _ could have stopped this whole mess before it even started.”

 

“Hm, you make a fair point,” Mephisto conceded, tilting his head. “But alas, what would it have changed? Your grandfather still would have gone through with his plan, with or without you-nothing could have stopped him at that point. Telling Rin would have just made him angry and emotionally unstable when he faced Satan, and had I attempted to tell either of you sooner, I doubt you would have believed me. Would you have trusted my word over that of the Vatican’s, or even your unknown grandfather’s?”

 

Rin scowled, sharing a look with Yukio. Mephisto rolled his eyes. “Now, now, none of that. I’m making it up to you now, aren’t I?”

 

“You are?” Yukio and Rin chimed.

 

“Settle down, have some tea, get comfortable,” Mephisto waved his hand airily. “This might take awhile.”

 

“Trust me, all we have is time,” Yukio said. 

 

The sun was overhead, bright morning light shining through the window. Rin had dark circles under his eyes, and Yukio was sure he was much the same. Sleep had been as elusive as answers the past few days, what with the revelation of demons to the general public and the destruction caused by the Gehenna Gate and Satan in Yukio’s body. Yukio missed Rin’s home cooked meals, even though he refused to admit the weakness. It was hardly the time for him to complain about army rations when people’s homes had been destroyed. 

 

Mephisto, on the other hand, seemed like he had never been better. There was a gleam in his eye, and a continual smile tugged at his lips. The strange green hamster Yukio didn’t even want to begin contemplating was happily running in a small hamster wheel, the squeaking gears making Yukio want to clamp his hands over his newly sensitive ears (no wonder Rin was so grumpy in the mornings, if this was what he had to wake up to).

 

In any case, the Chairman leaned back in his chair, staring at the duo like he was sizing them up. Yukio felt himself unintentionally straighten, feeling Rin do the same next to him. Mephisto grinned, taking a sip of tea and letting the expensive China clink as he set it down.

 

“Very well. When your mother and Satan, well, brought you two into existence, it wasn’t exactly, shall we say,  _ conventional _ . As you know, Satan could not possess a human body other than your mother’s, and had to, ahem, procreate another way,” Mephisto glanced at the pair, sliding a folder across the desk. 

 

Rin grabbed it, leaning over so Yukio could examine the files. Pushing up his glasses, Yukio scanned the document, steadfast ignoring the Research Lab for Artificial Life logo on some of the older papers.  DNA tests, gene analysis, comparison to human genes. It looked like they had analyzed their blood from every possible prospective.

 

“He let his powers flow through her,” Mephisto continued, “by drawing on the life force of Gehenna. But humans and demons were never meant to mix, and the power had to make a few biological changes to your DNA. Obviously, your DNA was mangled, though until recently we had no idea the extent at which it was.”

 

Nodding at the pair, Mephisto fed his hamster another lolipop, pointing out two double Helices printed next to each other. “You can thank your grandfather for so carelessly spilling your blood all over the place. Had anyone but the Vatican scientists obtained it, you might have gotten something much worse than test results.”

 

Yukio frowned. Last he’d seen, the Poland laboratory was destroyed, but if Mephisto was saying what he thought- 

 

“You may have started off from the same embryo, but each of your DNA strands were individually customized by the combination of Satan’s powers and Gehenna’s life force,” Mephisto continued. He looked rather delighted by the prospect. Yukio tensed, feeling Rin do the saem beside him. 

 

Mephisto smiled, sipping his tea. “Technically, neither of you have parents.” 

 

Yukio sat back in the chair, hard. He had wondered, looking over the sheets, but outright confirmation was just. He didn’t really know how to feel. Rin had no such problems expressing his feelings.

 

“W-what do you mean, you jerk?” Rin shouted, springing out of the chair. His tail swung about agitatedly, nearly knocking Yukio’s glasses off his nose. “We wouldn’t exactly be here without them-stop messing with our heads!”

 

“Oh, but I’m not,” Mephisto tilted his head. “Honestly, now you’re just making me repeat myself. By science’s standards, neither of you are related to anyone of this world, or of Gehenna. Then again, demons don’t really have genetic material. Of course, since the two of you are hybrids, you had to start somewhere, but in the process of your creation, Satan’s flames created two unique children-the two of you! However, unlike human DNA, in which the parents and the child’s DNA are different but related, the two of you have absolutely nothing in relation.”

 

Rin looked rather pale, fists trembling at his sides. Yukio grabbed his arm and pulled him back down, a sort of numbness spreading throughout his body.

 

“Well, I suppose you could consider yourself spawn of Gehenna, but with no relation to each other,” Mephisto continued, not looking the least bit concerned. “Did you ever wonder why you two looked nothing alike?”

 

“We’re fraternal,” Yukio protested weakly.

 

“Twins, brothers. Could even be lovers,” Mephisto winked slyly. 

 

Yukio felt his face darken. Beside him, Rin spluttered, falling out of the chair. And, yep, the numbness was gone, but Yukio wondered if he preferred it to the embarrassed heat spreading throughout his body.

 

“Excuse me?” Yukio asked indignantly.

 

“You may have been born of the same mother, but you are no more brothers than Shura is a sister to you. The only thing the two of you share is a last name. Oh, and a surprising compatibility concerning your blue flames.” Stroking his chin, Mephisto glanced critically between the pair. “Ah, I wonder. What would such a bond do to your special,  _ special _ powers? You had such a beautiful combination with this brotherly bond, but as  _ lovers- _ ”

 

“Y-Yukio and I are brothers!” Rin burst out, face flaming red. He hunched over as he crawled back onto the chair besides Yukio, crossing his arms defensively. 

 

Mephisto shrugged. “If you choose to be. Really, it’s none of my business what you two do with yourselves.”

 

“We’re not doing anything,” Yukio hissed, feeling claws digging into his palms. He took several deep breaths, forcing his demonic side down. One of the hairs on Rin’s head was on fire, and Yukio pinched out the blue flame. 

 

“But you could be,” Mephisto shrugged, taking far too much pleasure at the way the two inched away from each other. 

 

“Sh-shut up, you little-“ Yukio slapped a hand over Rin’s mouth, his own red complexion matching the elders.

 

“Thank you, we really appreciate you telling us this,” Yukio managed, left eye twitching. 

 

The demon shrugged, rising to his feet. The green hamster abandoned its wheel and jumped onto his arm, crawling up to his shoulder and almost sneering at the two of them from his perch. Yukio shook his head, dispelling the rather strange image.

 

“Hm. Either way, I do believe I have told you more than enough-all that I know, in any case. Have fun, little demons,” Mephisto waved over his shoulder, letting the door close behind him with a snick.

 

Lips pressed in a thin line, Yukio glanced at the files still splayed across Mephisto’s desk. Numbers and test results were scrawled across the papers, the same photo their grandfather had had in his locket paperclipped to a folder. 

 

Determidley avoiding Rin’s gaze, Yukio shoved the files into the folder, grabbing his no-longer-twin by the arm and dragging him out of the office.

 

“Yu-Yukio, wait!” Rin dug his heels into the ground, hand clasping Yukio’s.

 

“What?” Yukio scowled, yanking his arm out of Rin’s grip and trudging further away from what was quickly becoming a real life nightmare.

 

“What do you mean ‘what’? That guy just sprouted a whole bunch of nonsense and you agreed with him!”

 

Yukio pinched his nose. “ _ That guy _ is the director of our school and one of the most powerful and knowledgeable exorcists out there.”

 

Dodging Rin’s swipe at the folder, Yukio flipped through it, a sick sort of feeling swirling in his stomach. 

 

“He’s also a  _ demon _ ,” Rin emphasized. “He’s probably lying to us.”

 

Yukio snorted. “I doubt he is.”

 

“Well, how would  _ you _ know? You’re always to busy kissing up to those guys-”

 

Yukio growled, swiping at Rin. The elder jumped back, back hunching as he snarled back. Feeling his claws digging into his palms Yukio startled, turning around and desperately willing them to go away.

 

“I don’t  _ know _ , Rin, I’m not an all knowing being,” Yukio hissed.

 

“Yeah? Because you sure act like you are!”

 

“C’mon, that’s not true! Calm down and think rationally.”

 

“You want me to calm down? Our grandfather, who you didn’t bother to tell me about, nearly killed both of us and all of Assia.”

 

“You really want to do this now?” Yukio asked calmly. Inside, the demon in him hissed, blue flames licking the corner of his vision. He hadn’t ever felt this out of control, save the first time he jumped off a building. He could still remember Shura sniggering about ‘chickens’, and wow, this really wasn’t the time to be going down memory lane.

 

“Ya, I wanna do this now,” Rin huffed. “You refuse to talk about it anywhere, not here or at home. Come on, Yuki, you’re my brother-er, not-brother brother. You can trust me.”

 

There was a strange sort of bubble in his chest that Yukio recognized as affection before he hurriedly stuffed it back down. Rin could joke all he wanted about Yukio being an icy Disney queen, but there was some wisdom in her philosophy. Not that Yukio would ever admit to taking advice from a Disney princess.

 

“Rin, we have class in five minutes, let’s just talk about it tonight.”

 

Rin threw his hands up in the air. “That’s what you said yesterday, and the day before that. You can stand being a few minutes to class, jerkface. No one's gonna care.”

 

“Well, I care, so let’s go,” Yukio turned his back, fully intending on pushing the matter aside despite the folder burning a hole in his hands.

 

A burst of blue flames sliced next to his face and Yukio ducked, rolling to the ground and kneeling with one hand on his gun. “Rin!”

 

“Oh, so now you wanna talk?” Rin scowled, sword firmly latched to his back despite the blue flames licking his form. “What, afraid if I beat you your perfect exorcist status will plummet?”

 

“You’re just jealous I was the Paladin before you,” Yukio replied unthinkingly, cursing himself as a ring of red circled Rin’s eyes.

 

“You four eyed mole face, you were given that title, you didn’t earn it!” Rin’s tail whipped irritably, knocking aside his bookbag.

 

“Say that again, I dare you,” Yukio growled. Rin snarled back, hand reaching for his sword.

 

“Urgh, what is it now?” Shura sighed, pushing apart the squabbling pair. Their cram school classmates trailed behind her, watching the scene with resigned acceptance. 

 

Yukio was vaguely reminded of Shiro’s dead eyed stare that meant he was ‘done with their sh-’, er, stuff. Ok, so Yukio didn’t like cursing, sue him for having a clean mouth. Rin couldn’t say the same, what with the amount of times visiting nuns had forcibly washed his mouth out with soap.

 

Rin pointed at Yukio. “He started it!”

 

“I  _ did not _ .”

 

“Well, I’m ending it,” Shura grabbed Rin’s arm, yanking him along with her. The elder whimpered like a kitten and it was sorta cute  _ stop judging him _ . “Enough, you two, honestly. You’d think you were an old married couple instead of twins.”

 

Rin’s face, having finally faded to its normal pale color, flared up. Yukio wondered if it was too late to throw himself off the roof of True Cross Academy. “H-Hey, Shura! Lemme go, I can walk on my own.”

 

“We’re gonna go train a little,” Shura ignored him. “Have fun with the kids, chicken.”

 

“Don’t call me chicken!” Yukio glowered, whirling around in a flare of robes as he stalked to the classroom. 

 

Shiemi bit her lip, trailing after him with careful steps. Izumo just sighed, following suit as Bon traded glances with Shima and Konekomaru. No matter what the Vatican said, the Okumeras were definitely a handful.

  
  
  
  


 

The training room was dark when Rin followed Shura in, light fixtures flickering and a batting cage still melted from Rin’s memorable first training session. Shura stopped in the center of a sparring ring, dropping to stretch. 

 

“What, no candles?” Rin asked curiously, searching for the brown bag that always seemed to pop out of nowhere during their training sessions.

 

“Not today,” Shura slid out her sword. “Today we’re gonna scope out your powers.”

 

Rin blinked. Pointed to himself. “Me?”

 

“Yes, you,” Shura rolled her eyes. “You and Yukio combined powers, something neither of you has done before. What with Yukio’s powers just awakening, I need to find out if anything has changed with your flames before we can focus on working on controlling your brother’s.”

 

“He’s not my brother,” Rin grumbled, unsheathing his sword and reveling in the feeling of flames dancing across his skin. “We don’t even have parents.”

 

Shura dropped her sword. “What?”

 

Rin looked up, confused at her shocked expression. “What?”

 

“What do you mean Yukio isn’t your brother?”

 

Rin felt his face heat up again and was glad that True Cross had yet to replace the broken light fixtures in the training room. “Uh. Mephisto just told us.”

 

Shura looked transfixed by the story, going so far as to put her sword away and sit on the floor. 

 

“Children of Gehenna, huh?” She muttered.

 

Rin shrugged helplessly. “Whatever we are, we aren’t brothers.”

 

Sighing, Shura placed a hand on his shoulder. “Look, you might not have the same blood running through your veins, but you’re family, ok? You’re his brother and he’s yours, or whatever the hell you want to call your relationship.”

 

“Why does everyone keep saying things like that?” Rin buried his face in his hands. At the rate this day was going, he doubted his face would ever return to its normal color.

 

“Well, lots of siblings can’t stand the sight of each other, and although you and Yukio fight a lot, you never leave each other behind. It’s the kind of relationship most couples would kill to have,” Shura said. “Although, I’ll be honest, I wasn’t expecting the old married couple comment to be taken so literally.”

 

“Mephisto pretty much told us we could be lovers,” Rin muttered. “I don’t think I’m ever gonna recover.”

 

Shura snorted, slapping a hand over her mouth. “I’m sorry, I shouldn’t laugh. But that’s kinda hilarious. Never saw the Chairman as a shipper.”

 

“You should see his manga collection,” Rin sighed, pulling out his own sword. “I’m sorry, could we not talk about this right now?”

 

“Sure kid,” Shura drew her sword, “we’ll talk later, after I kick your ass. Now, give me everything you’ve got!”

 

Rin grinned, feeling his fangs poke at the edges of his lips. “Wouldn’t dream of doing any less.”

  
  
  


 

Shiemi shrieked as the classroom shook for the eighth time in the last few minutes, her pencils rolling off the desk. Bon was glaring at the ceiling like he was blaming it for the pencil skids across his notes. Izumo and Shima seemed content to ignore the disturbance. Konekomaru would have asked their teacher what was happening, but the half-crazed glean in Yukio’s eyes kept his hands firmly on the desk.

 

Yukio focused on the lecture with an oppressive intensity. His left eye was twitching even as he lectured, chalk screeching on the chalkboard that had the ExWires wincing. His hands were shaking, and he kept side-eyeing a file placed carefully on his desk.

 

Finally, Izumo stood, packing her bags. She stalked up to Yukio’s desk and slammed her hands down on the wood.

 

Yukio jumped, seeming to have just noticed her appearance. “Izumo?”

 

“I’m leaving,” she announced. “There’s obviously something on your mind and I haven’t been able to understand you for the last ten minutes. Go fix it.”

 

Yukio blinked. “Class is in session, Kamiki. Please go sit down.”

 

The classroom rumbled again and Yukio startled, like he had just noticed the dust raining down from the ceiling. “What was that?”

 

“Bye,” Izumo called, pigtails trailing behind her as she slipped through the door.

 

Bon sighed, packing his bags. 

 

“Bon!” Konekomaru yelped, scandalized. 

 

“As much as I hate to admit it, she’s right,” Bon said. “Mister Okumura, I’ll do some extra reading tonight. I think you need to get whatever it is that’s freaking you out sorted out. We’re not learning much we don’t know already, anyways.”

 

Shima shrugged, following suit. “C’mon, Shiemi, Konekomaru and I can help you find a new spirit for you to tame.”

 

“O-oh,” Shiemi blinked. “That’s alright, I think we should stay and-”

 

“I need help with a few of the plants in here,” Bon interrupted, looking like it pained him to say it. 

 

Konekomaru looked incredulous, but Shiemi lit up, her bag packed in seconds. 

 

“Let’s go! I can show you the new Mr. Sancho we got last week-”

 

“I think I’ll go take a nap,” Shima yawned, stretching his arms above his head and patting Yukio on the arm as he passed. “Don’t think too hard, teach, or you’re gonna give yourself an ulcer.”

 

“Hey, wait for me!” Konekomaru frantically shoved his notes into his bag before speeding after his other classmates.

 

The door shut with a slam, and then the only two people left were Yukio and Nemu-the-sock-puppet-kid.

 

Nemu and Yukio stared at one another. 

 

“I think you’re being stupid and should go face your problems,” Nemu’s pink bunny puppet told Yukio, and the student walked out the door without a backwards glance. 

 

“...What is happening?”

  
  
  


 

 

“Great-huh-job, kid. I think I actually broke a sweat this time,” Shura grunted, leaning on her sword.

 

Rin huffed, sheathing his sword and flopping on the floor. “I think you broke my everything.”

 

“Stop being dramatic,” Shura rolled her eyes, stumbling to her feet. “If we were fighting for real, you would have beaten me. I’m pretty sure your powers have increased, by the way. The fireballs were new.”

 

“You think?” Rin huffed a laugh. “I think they scared me as much as they scared you.”

 

“Mm, really glad the True Cross was planning on remodeling this old place anyways,” Shura glanced at the smoking ruins, dust trickling down from the ceiling. 

 

Rin shrugged, picking up his sword. “I don’t feel any different, except.”

 

“Except?” Shura prompted.

 

“It’s gonna sound weird,” Rin groaned, glaring at his smoking sleeve until the flame went out.

 

“Weirder than the son of Satan becoming an exorcist?” 

 

Rolling his eyes, Rin trudged over to Shura and flopped down again. “I guess not. It’s just, when Yukio and I combined our powers, it was like my flames truly belonged to me. The flames responded to me-to us, and they followed our commands. It was as easy as breathing.”

 

Shura scanned him critically. Rin fidgeted uncomfortably, holding the silence for another minute before bursting out.

 

“Ok, what is it?! Stop staring at me like that, it makes me uncomfortable.”

 

“Hm? Oh yeah, sorry. Just had a thought,” Shura answered absently, eyes catching on a strange symbol spiraling out the top of his shirt. She nodded to it. “Was that always there?”

 

“Was what-argh!” Rin scrambled to his feet, pulling on the shirt collar to reveal even more black lines. Nearly strangling himself with his tie, Rin yanked at the shirt, turning away from Shura. 

 

The ink spread across the right side of his chest, swirls and patterns a dark blue. 

 

“I don’t remember getting a tattoo?”

 

Shura snorted. “Too much to drink?”

 

“I don’t  _ drink _ .”

 

“Well that thing came from somewhere. Got a better explanation?”

 

Yukio burst through the door, eyes catching on Rin’s strange tattoo. “Why does that look exactly like mine?”

 

Rin blinked, finally noticing the blue ink spiraling over the top of Yukio’s collar. “Well that’s just fantastic.”

 

“Couples tattoos?” Shura clicked her tongue. “I expected better, you two.”

 

“Shura!”

 

“I would’ve thought the chicken would’ve chickened out.”

 

“ _ Don’t call me chicken _ .”  
  



End file.
